The present application claims priority to Japanese Application No. P11-317109 filed Nov. 8, 1999, which application is incorporated herein by reference to the extent permitted by law.
1. Field of the Invention
The present invention relates to a solid image pickup device comprising a transfer electrode having a shunt wiring layer formed thereon, and a process for producing the same.
2. Description of the Related Art
Polycrystalline silicon is generally used as a transfer electrode of a solid image pickup device.
Because polycrystalline silicon of the transfer electrode has high resistance, propagation delay is caused, and therefore it is difficult to realize high-speed operation of the image pickup device and an image pickup device having a large area.
In order to solve the problems, such a constitution has been proposed that a wiring layer formed with a metal having low resistance, i.e., a so-called shunt wiring layer, is formed on the transfer electrode through a dielectric film, and the shunt wiring layer is connected to the transfer electrode via a contact part. Because the propagation occurs through the shunt wiring layer of low resistance by using the constitution, high-speed operation can be realized.
A cross sectional view of the vicinity of a photo sensor unit of a CCD solid image pickup device is shown in FIG. 6 as an example of a solid image pickup device using a shunt wiring layer.
The CCD solid image pickup device 50 comprises a semiconductor substrate 51 having formed thereon a photo sensor unit containing a photodiode, a vertical charge transfer unit transferring a charge, a readout unit conducting readout a signal charge between the photo sensor unit and the vertical charge transfer unit and a channel stop region separating from the adjacent pixels, and a transfer electrode 53 is formed on the region other than the photo sensor unit through a dielectric film 54. FIG. 6 shows a cross section of a part where two of the transfer electrodes 53 overlap each other through the dielectric film 54.
A shunt wiring layer 56 formed with a metal, such as aluminum and a high melting point metal, is arranged on the transfer electrode 53 through the dielectric film 54.
A buffer layer (buffer wiring) 55 is provided between the shunt wiring layer 56 and the transfer electrode 53, and the buffer layer 55 is connected to the transfer electrode 53 and the shunt wiring layer 56, respectively, via contact parts not shown in the figure.
A light shielding layer 57 covering the entirety is formed on the shunt wiring layer 56 through the dielectric film 54. The light shielding layer 57 has an opening 52 formed on the photo sensor unit and is formed to cover the image pickup region other than the opening 52.
Furthermore, an on-chip lens 59 is formed on the light shielding layer 57 through a dielectric layer 58 having a smoothened surface, so as to condense incident light, which is directed to the opening 52.
In the CCD solid image pickup device 50, the shunt wiring layer 56 is connected to the transfer electrode 53 via the buffer layer 55 to lower the resistance of the transfer electrode 53, whereby the propagation velocity is improved.
However, when a high melting point metal, such as tungsten, is used in the shunt wiring layer 56, such a problem is caused by a heat treatment conducted after the formation of the dielectric film on the light shielding layer 57 that the contact resistance between the shunt wiring layer 56 and the transfer electrode (accumulation electrode) 53 is increased in the constitution where the shunt wiring layer 56 and the transfer electrode 53 is directly connected without the buffer layer 55, and the contact resistance between the shunt wiring layer 56 and the buffer layer 55 is increased in the constitution shown in FIG. 6.
The increase in contact resistance is mainly caused by the following factor. The polycrystalline silicon of the buffer layer or the transfer electrode and the high melting point metal are reacted by the heat treatment to form a silicide layer at the contact part, and the volume is increased in forming the silicide layer to lift the shunt wiring layer 56, so as to form a gap at the contact part with the buffer layer or the transfer electrode.
On the other hand, when Al is used in the shunt wiring layer 56, the heat treatment cannot be conducted at a high temperature because the Al is melted when a heat treatment at a high temperature is conducted after the formation of the dielectric film on the light shielding layer 57.
Therefore, defects caused by damages of the silicon substrate 51 received in pattering the light shielding layer 57 cannot be sufficiently recovered, so as to increase the dark current.
The dielectric layer on the shunt wiring layer 56 is necessarily a dielectric film that can be formed at a low temperature, and in order to sufficiently ensure the coverage and the withstand voltage of the dielectric film, it is necessary to form the dielectric film with a large thickness. Therefore, such a problem is caused that the film thickness from the surface of the silicon substrate 51 to the upper end of the light shielding layer 57 is increased to lower the utilization efficiency of light, whereby the sensitivity is deteriorated.
In order to solve the problems associated with the conventional techniques, an object of the invention is to provide a solid image pickup device that can be operated at high speed and can suppress the dark current with high sensitivity, and a process for producing the same.
The invention relates to a solid image pickup device comprising plural sensor units formed on a substrate, a transfer electrode provided among the sensor units on the substrate, and a shunt wiring electrically connected to the transfer electrode, the shunt wiring comprising a high melting point metal layer and a layer comprising a nitride or an oxide of a high melting point metal.
The invention also relates to a process for producing a solid image pickup device comprising a step of forming a transfer electrode on a substrate having sensor units formed thereon, a step of forming, on the transfer electrode, a shunt wiring layer comprising a layer comprising a nitride or an oxide of a high melting point metal and formed thereon a high melting point metal layer, and a step of conducting, after forming a dielectric film to have concave parts on the sensor units, a heat treatment at a temperature of from 800 to 900xc2x0 C. or a heat treatment to subject the dielectric film to reflow.
According to the constitution of the invention, because the shunt wiring of the transfer electrode is formed with an accumulated film of a high melting point metal and a nitride or an oxide of a high melting point metal, the resistance of the transfer electrode is lowered by the shunt wiring, and a heat treatment at a high temperature can be conducted owing to the use of the high melting point metal.
According to the production process of the invention, because the heat treatment is conducted at a temperature of from 800 to 900xc2x0 C., defects on the surface of the substrate can be recovered.
Furthermore, because the shunt wiring layer is formed with the accumulated film comprising a nitride layer or an oxide layer of a high melting point metal having a high melting point metal layer formed thereon, the reaction between the transfer electrode and the shunt wiring layer on conducing the heat treatment can be suppressed, and the increase of the contact resistance to the transfer electrode is suppressed to decrease the resistance of the transfer electrode.